It depends how heavy the fitting is. If it is light, you'll be able to drill the plasterboard and insert toggle fittings.
However, if it is heavy (such as anything hanging from a chain, or a fan or heavy glass shade), you must locate the ceiling joists and screw into these. If the joists are not in the right position, you'll need to pull down a section of ceiling to install a cross beam (called a noggin) that you can mount to and then restore the ceiling.
You can't use normal rawlplugs. They are for masonry fixing. However, the same company makes lots of fittings, including the toggles mentioned above.
Christian.